During your visit to dream castle in the zil desert go into the cellar where the zombies are
If you follow the wall around to the east kill the zombies ( will be easier this way ) and the lizards if you wish to then on the east wall will be holes with spikes what shoot out and give you damage ( they will not kill you ) then all you need to do is get one of your characters to use the spell heal over and over again and in around 30 min to 1 hour your water level will be 20+ ( when you get 20 you unlock the best party heal spell )